Etalons, also known as Fabry-Pérot interferometers, are commonly used in telecommunications, lasers, spectroscopy and astronomy. By way of non-limiting example, etalons may be used to control and/or measure the wavelengths of light. In particular, an etalon may be used to filter all but only a very narrow bandwidth of light.
Commonly, etalons comprise a pair of parallel optical plates spaced apart by a fixed distance. The spacing distance is typically on the order of 1 mm or several millimeters. The opposing surfaces of the optical plates that face each other are commonly reflective or partially reflective. The facing surfaces may have reflective or partially reflective coatings applied thereto. Often, the optical plates are wedge shaped (i.e. their proximate surfaces are parallel to one another, but their distal (or rear) surfaces are provided at an incline relative to the proximate surfaces) to prevent or minimize the distal surfaces from producing interference fringes. The distal surfaces may have anti-reflective coatings.
Illumination for an etalon is typically provided by a diffuse light source located at the focal plane of a collimating lens. As a light ray passes through the paired optical plates, the light ray is multiply reflected between the paired optical plates to produce multiple transmitted light rays. The multiple transmitted light rays may be collected by a focusing lens and directed onto a screen to produce an interference pattern. The interference pattern may take the appearance of a set of concentric rings due to constructive and destructive interference of the light rays.
The sharpness of the rings and/or resolving power of an etalon may be affected by the quality (e.g. reflectivity, smoothness and/or flatness) of the reflecting surfaces. A high reflectivity surface (e.g. a surface with relatively high reflectivity) tends to produce a set of narrow bright rings against a dark background while a low reflectivity surface (e.g. a surface with relatively low reflectivity) tends to produce wider bright rings. An etalon with relatively high reflectivity surfaces which produce narrow interference rings is said to have high finesse while an etalon with relatively low reflectivity surfaces which produce wider interference rings is said to have low finesse. High finesse is a generally desirable quality of an etalon because high finesse increases the ability to resolve different rings from one another and the corresponding resolving power of the etalon. In a perfect etalon (with perfectly parallel and perfectly flat reflective surfaces), the finesse of an etalon depends on the reflectivity of the reflective surfaces, with higher reflectivity surfaces resulting in higher finesse. In real etalons, however, imperfections in surface flatness and parallelism tend to reduce contrast between rings and, consequently, lower the resolving power of the etalon due to multiple reflections in the etalon.
Traditionally, to achieve high finesse, the opposing surfaces of the optical flats each are polished to achieve an extremely high degree of surface flatness or a correspondingly low surface figure. For example, opposing surfaces of the optical flats used in typical etalons are often polished to have surface figures of less than approximately λ/20, where λ corresponds to the light source with which the etalon is to be illuminated). In a typical scenario, where the light source used to illuminate the etalon is a HeNe laser having a wavelength of 633 nm, this surface figure corresponds to maximum irregularities on the order of approximately 32 nm. Manufacturing glass plates, such as optical flats, having such a low surface figure can be very time consuming and expensive and may be especially difficult for thin optical plates, as used in traditional etalons.
Prior art etalons are typically made of an optical material such as quartz crystal because quartz exhibits a relatively low coefficient of thermal expansion. Quartz can be very expensive to use in such applications and increases the costs of manufacturing traditional etalons. Less expensive materials such as borosilicate glass (also known as Pyrex™) exhibit a relatively high coefficient of thermal expansion and are not suitable for traditional methods of making etalons. Because of the desirability of flatness of the proximate surfaces (e.g. surface figure on the order of λ/20 or less, as discussed above), the optical plates used to make etalons using traditional techniques are often relatively thick (e.g. over 5 mm) to achieve sufficient surface figure while avoiding bending or deformation. Plates with such thicknesses involve correspondingly high material costs relative to plates made with relatively low thicknesses. In general, fabricating surfaces with low surface figure may be difficult and expensive.
There is a general desire for improved and inexpensive methods to manufacture etalons.
The foregoing examples of the related art and limitations related thereto are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.